Golf balls are typically categorized into two groups: Solid and wound. Solid golf balls commonly include a core encased by a cover. On the other hand, wound golf balls are generally constructed from a liquid or solid center encased by tensioned elastomeric material and a cover. The core in solid golf balls may be either single layered or have multiple layers. The cover may also be single or multi-layered. Sometimes, an intermediate layer is disposed between the core and the cover.
Golf ball manufacturers tailor characteristics such as initial velocity, spin, feel, resilience, compression, durability, flexural and/or inertial properties in order to maximize a golfer's performance. Variations in weather conditions, terrain as well as individual playing styles or abilities remain factors which make it desirable for manufacturers to have cores which exhibit a wide range of properties. While hard golf balls provide high initial velocity and therefore increased distance, these balls also have a relatively lower spin rate than a softer ball, which makes them more difficult to control. Accordingly, it is desirable to provide a golf ball which provides the benefits of a harder ball without sacrificing control.
Golf ball cores and/or centers are typically constructed with a thermoset rubber, such as a polybutadiene-based composition. The cores can be heated and crosslinked to create certain characteristics, such as higher or lower compression, which can impact the spin rate of the ball and/or provide better “feel.”
Heretofore, most single core golf ball cores have had a conventional hard-to-soft hardness gradient from the surface of the core to the center of the core. The patent literature contains a number of references that discuss a hard surface to soft center hardness gradient across a golf ball core.
U.S. Pat. No. 4,650,193 to Molitor et al. generally discloses a hardness gradient in the surface layers of a core by surface treating a slug of curable elastomer with a cure-altering agent and subsequently molding the slug into a core. This treatment allegedly creates a core with two zones of different compositions, the first part being the hard, resilient, central portion of the core, which was left untreated, and the second being the soft, deformable, outer layer of the core, which was treated by the cure-altering agent. The two “layers” or regions of the core are integral with one another and, as a result, achieve the effect of a gradient of soft surface to hard center.
U.S. Pat. No. 3,784,209 to Berman, et al. generally discloses a soft-to-hard hardness gradient. The '209 patent discloses a non-homogenous, molded golf ball with a core of “mixed” elastomers. A center sphere of uncured elastomeric material is surrounded by a compatible but different uncured elastomer. When both layers of elastomer are concurrently exposed to a curing agent, they become integral with one another, thereby forming a mixed core. The center of this core, having a higher concentration of the first elastomeric material, is harder than the outer layer. One drawback to this method of manufacture is the time-consuming process of creating first elastomer and then a second elastomer and then molding the two together.
Other patents discuss cores that receive a surface treatment to provide a soft ‘skin’. For example, U.S. Pat. No. 6,113,831 to Nesbitt et al. generally discloses a conventional core and a separate soft skin wrapped around the core. This soft skin is created by exposing the preform slug to steam during the molding process so that a maximum mold temperature exceeds a steam set point, and by controlling exothermic molding temperatures during molding. The skin comprises the radially-outermost 1/32 inch to ¼ inch of the spherical core. U.S. Pat. Nos. 5,976,443 and 5,733,206, both to Nesbitt et al., disclose the addition of water mist to the outside surface of the slug before molding in order to create a soft skin. The water allegedly softens the compression of the core by retarding crosslinking on the core surface, thereby creating an even softer soft skin around the hard central portion.
Additionally, a number of patents disclose multilayer golf ball cores, where each core layer has a different hardness thereby creating a hardness gradient from core layer to core layer.
There remains a need, however, to achieve a single layer core that has a generally soft-to-hard gradient (a “negative” gradient), from the surface to the center with varying hardness, and to achieve a method of producing such a core that is inexpensive and efficient. A core exhibiting such characteristics would allow the golf ball designer to create products with unique combinations of compression, “feel,” and spin.